Composition in gel form comprising carbon nanotube and ionic liquid and method for production thereof

ABSTRACT

Disclosed is a novel technology, by which carbon nanotubes can be easily worked without degrading the characteristic properties thereof. A gel composition including a carbon nanotube and an ionic liquid is produced by pulverizing, in the presence of the ionic liquid, the carbon nanotube by applying a shearing force thereto, and then, if needed, subjecting the product of the pulverization to centrifugal separation. The gel composition exhibits an excellent workability and can be worked simply by forming a desired shape by subjecting the composition in a fluidized state to application of an external force by such an operation as a printing, coating, extrusion or injection operation, and then removing the ionic liquid with a solvent or an absorbent.

FIELD OF TECHNOLOGY

The present invention belongs to the field of molecular nanotechnologyand, particularly relates to a novel material with a high workabilityderived from carbon nanotubes.

BACKGROUND ART

Carbon nanotubes have attracted considerable attentions as a leadingmaterial of the next generation, as they exhibit excellent electricproperties diversely ranging from a metallic property to asemiconductive property, as well as a large surface area and a highmechanical strength. Thus, world-wide studies are being conducted onpractical uses thereof in a variety of fields, as electrical orelectronic materials, materials for reinforcing functional resins andthe like.

However, carbon nanotubes are not present in the form of individualtubes separated from each other but are present in the form of largebundles, which causes poor workability and is hence a great barrier tothe practicability of carbon nanotubes. Methods have been proposed forimproving the workability, in which carbon nanotubes are subjected to achemical treatment of the surfaces thereof so to give them improveddispersibility. However, it has been pointed out that the methods areproblematical because the treatment degrades the intrinsiccharacteristic properties of carbon nanotubes.

The purpose of the present invention is to provide a novel technology,by which carbon nanotubes can be easily worked without damaging thecharacteristic properties thereof.

DISCLOSURE OF THE INVENTION

Through extensive studies to achieve the above-mentioned object, thepresent inventors found that the utilization of an ionic liquid resultsin compositions or composites having an extremely excellent workabilityor processability and accomplished the present invention based on thisdiscovery.

Thus, according to the present invention, there is provided a gelcomposition comprising a carbon nanotube and an ionic liquid.

The present invention also provides a method for producing such gelcomposition comprising a carbon nanotube and an ionic liquid, whichcomprises a step of pulverizing, in the presence of the ionic liquid,the carbon nanotube by applying a shearing force thereto, and preferablya step of subjecting the product of the pulverization to centrifugalseparation.

According to the present invention there is further provided a methodfor working the gel composition comprising a carbon nanotube and anionic liquid, which comprises a step of forming a desired shape fromsaid gel composition by subjecting the composition in a fluidized stateto application of an external force by a printing, coating, extrusion orinjection operation, and a step of removing the ionic liquid from saidgel composition by bringing said shape in contact with a solvent capableof dissolving the ionic liquid or an absorbent capable of absorbing theionic liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows chemical structures of examples of ionic liquids suitablefor use in the present invention.

FIG. 2 shows a transmission electron microscopic (TEM) view of a gelcomposition of the present invention comprising a carbon nanotube and anionic liquid (a) in comparison with a TEM view of the carbon nanotubeprior to the formation of the gel composition (b).

FIG. 3 shows an electron absorption spectrum (a) and a Raman spectrum(b) of a gel composition of the present invention.

FIG. 4 shows the results of dynamic viscoelasticity measurement on a gelcomposition of the present invention.

FIG. 5 shows the results of differential scanning calorimetry (DSC) andX-ray diffraction (XRD) carried out on a gel composition of the presentinvention.

FIG. 6 shows the results of DSC and XRD carried out only on an ionicliquid, for purpose of comparison.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, there is provided a uniquetechnology which enables, simply by physical operations, the preparationof a carbon nanotube-containing material (composition) having anextremely excellent workability, wherein three factors (a carbonnanotube (1) and an ionic liquid (2) are pulverized under a shearingforce (3)) are indispensable and no gel compositions of the presentinvention can be produced in case that even one of the factors is notpresent.

More specifically: (1) A simple admixing of a carbon nanotube and anionic liquid without applying a shearing force will not produce the gelcomposition. (2) Use of carbonaceous materials other than carbonnanotubes, such as graphite, C₆₀, and active carbon will not produce thegel composition. (3) Pulverization of a carbon nanotube under a shearingforce using an ordinary organic solvent or an ionic liquid precursor,instead of an ionic liquid, will not produce the gel composition (cf.Comparative Examples set out later).

Thus, in producing a gel composition comprising a carbon nanotube and anionic liquid in accordance with the present invention, the carbonnanotube is pulverized, in the presence of the ionic liquid, by applyinga shearing force thereto in the first place.

The examples of means for applying a shearing force in the step of thepulverization include, but are not limited to, a manual or automaticgrinding operation in a mortar, in the case of a small-scale productionas in a laboratory, and the use of a wet milling machine with a highshearing force such as a ball mill, a roller mill or an oscillatingmill, in the case of a mass production. A kneader type of device mayalso be used. While the time required for the pulverization step is notlimited and varies depending upon the degree of the pulverization forthe respective uses, it is generally about from five minutes to onehour.

Through the step as mentioned above, there is obtained a black, pastyproduct. While the black pasty product may be used as it is, as acomposition of the present invention, in general it is preferablysubjected to centrifugal separation. Thus, the surplus ionic liquidwhich was not involved in the formation of the gel composition isremoved by the centrifugal separation.

Although the formation mechanism and structure of the gel compositioncomprising a carbon nanotube and an ionic liquid of the presentinvention have not yet been completely elucidated, a rough idea based onthe results of various analyses is as follows (see the Working Examplesset out later):

(1) The pulverization treatment under a shearing force will not cause achemical change of carbon nanotubes but cause a physical configurationalchange in which the degree of entanglement among the individual carbonnanotubes is decreased so as to form thinner bundles.

(2) It is thought that the gel formation is not due to the entanglementof carbon nanotubes, but is caused by the formation of a crosslinkedstructure (three-dimensional network structure) in which the ionicliquid molecules, attached to the surfaces of the less entangled carbonnanotubes through the “cation-π” interaction, serve to combine thebundles of carbon nanotubes with one another through ionic bonding.

It is well known that an ionic liquid as used in the present inventionis also referred to as a cold molten salt or simply as a molten salt,and is defined as a salt which assumes a molten state in a wide range oftemperatures including ordinary temperature (room temperature).

While a variety of known ionic liquids can be used in the presentinvention, it is preferred to use an ionic liquid which is stable andassumes a molten state at ordinary temperature (room temperature) or ata temperature very near ordinary temperature. As ionic liquids suitablefor use in the present invention there can be exemplified the onescomposed of a cation selected from the general formulae (I) through (IV)given below (preferably a quaternary ammonium ion) and a anion (X⁻).

In formulae (I) through (IV), R represents an alkyl group having ten orless carbon atoms or an alkyl group containing an ether bond or bondsand having ten or less carbon atoms plus hydrogen atoms. In formula (I),R¹ represents an alkyl group having 1-4 carbon atoms or hydrogen atom,which is preferably methyl group having one carbon atom. In formula (I),it is preferred that R and R¹ are not the same. In formulae (III) and(IV), x is an integer of 1-4.

Examples of the anion (X⁻) include at least one selected from amongtetrafluoroborate, hexafluorophosphate, bis(trifluoromethylsulfonyl)imidate, perchlorate, tris(trifluoromethylsulfonyl) carbonate,trifluoromethanesulfonate, dicyanamide, trifluoroacetate, an organiccarboxylate, and halogen ions.

As well known, a carbon nanotube, as used in the present invention, is acarbonaceous material of graphene sheet(s) rolled into a cylindricalform and carbon nanotubes can be divided broadly into single-walledcarbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs),depending upon the number of the surrounding walls, and also classifiedinto chiral (spiral) type, zigzag type and armchair type, depending uponthe structure of the grapheme sheet. Thus, a variety of carbon nanotubesare known. While the present invention can be applied to any type of theknown carbon nanotubes, it is generally easy for a single-walled carbonnanotube, which has a high aspect ratio (i.e., is fine and long), toform a gel and the present invention is therefore suitable for producinga gel composition from a SWCNT. Examples of carbon nanotubes suitablefor practical use include, but are not limited to, HiPco (commerciallyavailable from Carbon Nanotechnologies Co.,), which can be produced on arelatively large scale from carbon monoxide.

The ratio of a carbon nanotube and an ionic liquid can be determined bya simple test: A sufficient amount of the ionic liquid is used over thecarbon nanotube, so that a clear ionic liquid is isolated when the blackpasty product (the carbon nanotube+the ionic liquid) is subjected to thecentrifugal separation step following the pulverization. While the ratiodepends upon the types of carbon nanotubes and ionic liquids used, it isgeneral that an ionic liquid is used in an amount of more than 100 timesby weight over a carbon nanotube.

As the purity of carbon nanotube is low, the gel formation ability willfall. Thus, it is preferred to use a carbon nanotube which is purifiedas highly as possible from impurities such as the catalyst residue.While it is general that a carbon nanotube with a purity of about 70% orhigher is preferably used owing to efficient gel formation, the purityof carbon nanotubes is optional, ranging from a high purity to arelatively low purity depending upon the application.

The gel composition of the present invention is a rare material composedof a fine dispersion of a carbon nanotube, and features nonvolatility,incombustibility and high thermal stability, which properties arederived from an ionic liquid.

The gel composition of the present invention, which comprises a carbonnanotube and an ionic liquid, is further characterized in that itassumes a fluid state when applied with an external force whereas itpossesses a shape-retention ability as it is.

Thus, the gel composition of the present invention, taking advantage ofsuch characteristic properties, can be subjected to a working processfor shaping, which comprises a step of forming a desired shape[including planar (two-dimensional) ones such as dots, lines, letters orcharacters, patterns, figures, or fibrous materials, or stereoscopic(three-dimensional) products] by printing, coating, extruding orinjecting the composition with an appropriate tool or device such as aninjector, a jet-spray printer, a bar coater or a spray coater, and thena step of removing the ionic liquid from the shape produced. Removingthe ionic liquid from the product with the desired shape is accomplishedby bringing the product in contact with a solvent (such as water or analcohol) capable of dissolving the ionic liquid (specifically, forexample, the product is immersed in the solvent for extraction or theproduct is washed with the solvent), or by bringing the product incontact with an absorber (for example, a filter paper or fabric) capableof absorbing the ionic liquid. Thus, the product composed of a carbonnanotube retains its shape. Therefore, the gel composition of thepresent invention has a wide range of expected applications, includingnew types of carbon nanotube-containing materials such as paintingmaterials, printing materials, coating materials, molding materials,electronic device materials provided with a semiconducting or metallicproperty, or micromedical device materials.

EXAMPLES

The features of the present invention will be explained morespecifically with reference to the following examples, which are not forrestricting the present invention.

Example 1 Preparation and use of Gel Composition

On pulverizing 1 part by weight of single-walled carbon nanotube (HiPco:available from Carbon Nanotechnologies Co., purity >95%) and 200 partsby weight of ionic liquid, 1-butyl-3-methyl imidazoliumtetrafluoroborate (BMIBF₄: cf FIG. 1), there was obtained a black, pastyproduct. The pasty product was subjected to centrifugation (9100 g,three hours) resulting in separation of a black gel compositioncontaining the ionic liquid and approx. 1% by weight of the carbonnanotube (HiPco), from a clear solution of the ionic liquid. In the samemanner, gel compositions were prepared by using other types of ionicliquids, i.e., 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄),1-hexyl-3-methylimidazolium tetrafluoroborate (HMIBF₄),1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide(BMITf₂N), and 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆)(cf. FIG. 1). In the case where EMIBF₄ or HMIBF₄ was used, there wasobtained a gel composition containing approx. 1% by weight of the carbonnanotube, as in the case of BMIBF₄. In the case of BMITf₂N or BMIPF₆,the gel formation was more efficient, and thus a gel composition wasobtained with only approx. 0.5% by weight of the carbon nanotube(HiPco).

When each of the gel compositions was placed in an injector, it could beextruded therefrom in the form of a thread, with which a picture wasdrawn on a plate. On bringing the drawn picture in contact with a filterpaper, the ionic liquid was absorbed into the filter paper while theblack picture was stably retained.

Comparative Example Use of Organic Solvents and Other CarbonaceousMaterials

For comparison, in place of the above-mentioned ionic liquid, there wasused dichlorobenzene, ethanol or N,N-demethylformamide (DMF), as aconventional organic solvent, or 1-methylimidazole, as a precursor ofthe ionic liquid, and the resultant mixture was subjected topulverization, as in Example 1, in an automatic mortar under a highshearing force at room temperature for more than two hours. However, nogels were formed.

In the same manner as in Example 1 pulverization treatment under a highshearing force was also carried out, where there was used, ascarbonaceous material, graphite (1-2 μm, Aldrich), active charcoal, orC₆₀ (purity:99.9%, TCl) in place of the carbon nanotube (HiPco). No gelformation was observed.

Example 2 Structural Analysis of the Gel Composition

(1) Electron Microscopic Observation and Optical Spectral Measurement:

FIG. 2 a shows a TEM (transmission electron microscopy) view of the gelcomposition as prepared in Example 1 composed of a dispersion of carbonnanotube (HiPco) in deionized water (the ionic liquid: BMIBF₄) asprepared in Example 1. For comparison, FIG. 2 b shows a TEM view ofcarbon nanotube (HiPco) having undergone only an ultrasonic treatmentwith ethanol and thus prior to the gel formation. It is seen that thepulverization treatment under the shearing force resulted in a decreasedentanglement among the carbon nanotubes to form thinner bundles.

FIG. 3 a shows an electron absorption spectral measurement of the gelcomposition sandwiched between quartz plates. There are observedelectron spectra, as conventionally reported with respect to thesingle-walled carbon nanotube (HiPco), i.e. the spectra at 730-1000 nmand 1100-1700 nm attributed to the semiconductive property of thenanotube as well as the spectra at 540-640 nm attributed to the metallicproperty of the nanotube. FIG. 3 a shows a Raman spectrum of the gelcomposition (excitation wavelength: 488 nm). There is observed the Ramanspectrum well known with respect to the carbon nanotube (HiPco): 1588cm⁻¹ and 201 cm⁻¹. It can be seen from these results that thepulverization treatment under the shearing force will not cause anychemical change of the carbon nanotube but bring about only a physicalconformational change thereof.

(2) Dynamic Viscoelasticity Measurement:

Pulverization was carried out on carbon nanotube (HiPco) 15 mg and ionicliquid (BMIBF₄) 2.0 mL in an automatic mortar at room temperature forone hour, followed by centrifugal separation (9,100 g×one hour). Theresultant gel composition was measured for dynamic viscoelasticity. Theresults are shown in FIG. 4.

There is observed a plateau region in G′(modulus of storage elasticity)in the case of a low strain applied (γ=0.01 or 0.1), suggesting theformation of an elastic network structure in the gel composition,whereas, in the case of a high strain applied (v=1.0), G′ and G″(modulus of loss elasticity) greatly change with the change of angularfrequency, suggesting destruction of the gel. As shown in FIG. 4, evenwith a considerably low strain applied (γ<1.0), the modulus of storageelasticity G′ depends upon the angular frequency. It is thus estimatedthat the network structure forming the gel is not due to a strong forcesuch as the entanglement of the carbon nanotubes but is caused byrelatively weak physical interactions.

(3) Thermal Analysis and X-Ray Diffraction (XRD) Measurement:

The gel composition as prepared in Example 1 containing 0.5% by weightof the carbon nanotube (HiPco) (the ionic liquid: BMITf₂N) was measuredfor differential scanning calorimetric (DSC) analysis and X-raydiffraction (XRD).

The results of the DSC analysis are shown in FIG. 5 a. FIG. 5 b showsthe result of XRD measurement with respect to the intermediate regionbetween the exothermal peak at −52° C. and the endothermal peak at −4°C. These results of the DSC analysis and the XRD measurement arecompletely different from the results of DSC analysis and XRDmeasurement singly for the ionic liquid BMITf₂N (FIGS. 6 a and 6 b),which forms a polycrystalline structure at a low temperature. It is alsoconfirmed that the carbon nanotube does not exhibit any X-raydiffraction. It is thus considered that the simple XRD pattern as givenby FIG. 5 b is due to a single mode of molecular arrangement of theionic liquid over a wide region. The d value, 4.60 Å, as given by FIG. 5b is virtually in agreement with the interplanar spacing, 4.53 Å, in theformation of an imidazolium ionic pair as reported with respect tocrystalline EMIPF₆.

It is estimated from the results as shown in the above (2) and (3) thatthe formation of gel in the gel composition of the present invention iscaused by a three-dimensional network structure composed of carbonnanotubes combined with one another through ionic bonding due to ionicliquid molecules, wherein the ionic liquid molecules are arrangedlyattached on the surfaces of the less entangled carbon nanotubes throughthe “cation-π” interaction.

Example 3

A gel composition was prepared in the same manner as in Example 1 exceptthat, in place of the carbon nanotube (HiPco) as used in Example 1,there was used a carbon nanotube of a lower purity containing 20% byweight of the metal catalyst residue. The black gel composition obtainedwas composed of the ionic liquid and ca. 2.5% by weight of the carbonnanotube.

Example 4

A gel composition was prepared in the same manner as in Example 1 exceptthat, in place of the carbon nanotube (HiPco) as used in Example 1,there was used a carbon nanotube of a lower purity produced by the laserprocess containing 30% by weight of graphite. The black gel compositionobtained was composed of the ionic liquid and ca. 1.5% by weight of thecarbon nanotube.

INDUSTRIAL UTILITY

As apparent from the foregoing description, there is obtained a gelcomposition composed of a carbon nanotube and an ionic liquid, by asimple process of pulverizing, in the presence of the ionic liquid, thecarbon nanotube under a shearing force applied thereto. The gelcomposition obtained exhibits an excellent workability and thus it canbe worked by a simple process of forming a desired shape by subjectingthe composition in a fluidized state to application of an external forceby a printing, coating, extrusion or injection operation, and thenremoving the ionic liquid with a solvent or an absorbent.

Therefore, the gel composition of the present invention has a wide rangeof expected applications, including new types of carbonnanotube-containing materials such as painting materials, printingmaterials, coating materials, molding materials, electronic devicematerials provided with semiconductor or metallic properties.

1. A gel composition formed by a method which comprises pulverizingcarbon nanotubes by applying a shearing force to a mixture consisting ofcarbon nanotubes and an ionic liquid, wherein the ionic liquid is a saltwhich assumes a molten state at or very near room temperature.
 2. Thegel composition as claimed in claim 1, wherein the carbon nanotubes aresingle-walled carbon nanotubes.
 3. The gel composition as claimed inclaim 1, the method by which it is formed further comprising a step ofsubjecting the product of the pulverization to centrifugal separation.4. The gel composition as claimed in claim 1, wherein the gelcomposition is capable of assuming a fluid state when an external forceis applied.
 5. A method for producing the gel composition of claim 1consisting of carbon nanotubes and an ionic liquid, which comprises astep of pulverizing, in the presence of the ionic liquid, the carbonnanotubes by applying a shearing force thereto.
 6. The method forproducing the gel composition as claimed in claim 5, further comprisinga step of subjecting the product of the pulverization to centrifugalseparation.
 7. A method for using the gel composition of claim 1, whichcomprises the step of forming a desired shape from said gel compositionby subjecting the composition in a fluidized state to application of anexternal force by a printing, coating, extrusion or injection operation,and then a step of removing the ionic liquid from said gel compositionby bringing said shape in contact with a solvent capable of dissolvingthe ionic liquid or an absorbent capable of absorbing the ionic liquid.